1. Field of the Invention
The present invention relates to a communication device and a communication system. The present invention can be applied to, for example, a communication system capable of collecting data transmitted from a number of communication devices to a designated collecting communication device.
2. Description of the Related Art
In the IEICE (Institute of Electronics, Information and Communication Engineers) Transactions on Communications 2005 E88-B (3), pp. 873-881 (Document 1) and IEICE Technical Report IN 2006, “Design, Installation and Evaluation of Synchronous Sensor Information-Gathering Systems in Sensor Networks Installed in Buildings” (Document 2), a sensor network is described in which a wireless multi-hop network is constituted with a number of sensor terminals and a base station. Periodically, sensor information is collected from all the sensor terminals to the base station. In a sensor network described in Document 1 and Document 2, sensor information is sequentially transmitted from sensor terminals at peripheral portions of the sensor network in accordance with a collection interval, and sensor terminals closer to the base station relay the sensor information, appending their own sensor information thereto. Thus, the sensor information can be efficiently gathered by the base station with little delay. Moreover, in the sensor network described in Document 1 and Document 2, each sensor terminal transmits sensor information synchronously with a signal which a transmission destination sensor terminal will produce for transmitting sensor information, at a time slightly earlier than the time at which the transmission destination sensor terminal will transmit the sensor information thereof. Accordingly, a power supply of a transmission/reception processing section of each sensor terminal is turned on for receiving and transmitting sensor information only for a short period in the sensor information collection cycle, and the power supply of the transmission/reception processing section can be turned off at other periods. Therefore, the sensor information can be gathered with efficient use of electrical power.
FIG. 5 shows an example of a sensor network in which the conventional sensor information collection system described in Document 1 and Document 2 is applied. A number assigned to each sensor terminal is a number of hops required for transmission of sensor information from that sensor terminal to the base station. FIG. 6 shows transmission/reception timings of sensor terminals ST4-2, ST3-2, ST2-3 and ST1-2 over time, which are on a path shown by a heavy line in FIG. 5.
A base station BS transmits beacon information designating timings for collection of sensor information, at the interval of collection of sensor information.
A sensor terminal ST1 can receive the beacon information transmitted by the base station BS. At a time earlier by a pre-specified offset interval than a timing at which the sensor terminal ST1 will receive the beacon information transmitted by the base station BS, the sensor terminal ST1 puts together sensor information sent from (an)other sensor terminal(s) ST2 up until that time with sensor information acquired from a sensor of the sensor terminal ST1, and transmits all the sensor information. The information that is transmitted includes the number of hops (1 in this case) required to reach the base station BS.
A sensor terminal which cannot receive the beacon information from the base station BS (ST2 is considered as an example) receives sensor information transmitted from surrounding sensor terminals and selects from them the sensor terminal with the smallest number of hops required to reach the base station BS (i.e., ST1). Then, similarly to the above-described sensor terminal ST1 which can receive the beacon information from the base station BS, at a time earlier by the pre-specified offset interval than the timing at which that sensor terminal (ST1) will transmit, this sensor terminal (ST2) puts together sensor information sent from (an)other sensor terminal(s) (ST3) up until that time with sensor information acquired from a sensor of this sensor terminal (ST2), and transmits all the sensor information. The information that is transmitted includes the number of hops required to reach the base station BS.
Each sensor terminal operates as described above, and thus transfers of sensor information are implemented at the timings shown in FIG. 6, and the base station BS can periodically collect sensor information from all the sensor terminals.
Here, it is sufficient that each sensor terminal can receive both sensor information that is transmitted at a timing earlier by the offset interval than a transmission timing of this terminal and sensor information that is transferred by a transmission destination sensor terminal at a time later to the offset interval than the transmission timing of this terminal. Accordingly, control is performed such that the transmission/reception processing section thereof is turned on only for this period and the transmission/reception processing section is turned off in other periods. Therefore, operation with low power consumption is possible.
IEICE Technical Report SN 2006-05 pp. 23-29, “A Fast and Reliable Transmission Mechanism of Urgent Information in Sensor Networks” (Document 3) illustrates a system for, in the above-described sensor network, collecting sensor information which is more urgent then usual with high reliability and little delay. When urgent information arises at a sensor terminal, the sensor information is transmitted with an urgent flag, which indicates that the information is urgent, appended thereto. A sensor terminal that receives sensor information with the urgent flag appended switches into an urgent state. In the urgent state, control to turn the transmission/reception processing section on and off is stopped and the transmission/reception processing section is set into a continuously on state, such that urgent sensor information can be transferred at any time. In addition, in order to improve transfer reliability of urgent sensor information, transmissions of sensor information other than sensor information that includes urgent information are suppressed. By such operations, as described in FIG. 7, a region for transferring urgent information with small delays and high reliability can be formed between the sensor terminal at which the urgent information arose and the base station.
FIG. 7 shows states of sensor terminals and the like in the sensor network when urgent information has arisen as described above. In FIG. 7, sensor terminal ST4-2 is the terminal at which the urgent information arose. This sensor terminal ST4-2 and sensor terminals ST3-2, ST3-3, ST2-3 ST2-4 and ST1-2, which are on shortest paths from the sensor terminal ST4-2 to the base station BS, form a region for prioritizing and transferring urgent information.
However, the system for collecting urgent information described in Document 3 has the following problem.
If a sensor terminal fails to receive urgent information due to an effect of noise, an impact or the like, this sensor terminal cannot switch into the urgent state. Therefore, on/off control of the transmission/reception processing section thereof continues. Consequently, the transmission/reception processing section of this sensor terminal is set to the off state until the next time for collecting usual sensor information. In this period, the sensor terminal cannot switch into the urgent state. As a result, an amount of time required to establish the region which can transfer the urgent information with high reliability and little delay is lengthened.
FIG. 8 shows changes of state of the sensor terminals when there is a failure in reception of urgent information. As shown in FIG. 8, if sensor terminal ST3-2, which is to transfer the urgent information, fails to receive the urgent information (at T1), timings at which this sensor terminal ST3-2 and sensor terminals ST2-3 and ST1-2, to the base station side of the sensor terminal ST3-2, switch to the urgent state are delayed by an amount corresponding to one cycle of the sensor information collection interval (T3). Urgent information which arises in this period (at T2) cannot be transferred to the base station BS.
Accordingly, a communication device and communication system are desired which, even when a relay of urgent information between communication devices fails, can transmit urgent information as quickly as possible to a collecting communication device which is a final transmission destination.